HISTORY A 21-year-old man initially presented to an outside orthopedic clinic complaining of 5–6 weeks of left knee pain. At the time of onset he recalls slipping and twisting his knee while playing basketball. He did not hear or feel a pop and was able to continue playing. He was initially diagnosed with a lateral collateral ligament sprain, but did not respond to conservative treatment. After three months of symptoms, he underwent an arthroscopy which revealed diffuse synovitis, which was treated with a partial synovectomy. Post-operatively, he continued to have diffuse knee pain prompting further work-up. During this time he traveled to Guatemala where his symptoms worsened. Upon follow-up in the United States, serologic and knee aspirate evaluation suggested a rheumatologic etiology, possibly consistent with a seronegative arthropathy. He was treated with NSAIDs, sulfsalazine and eventually methotrexate over a four month period without significant improvement in his symptoms. With continued lateral knee pain and intermittent swelling, he presented for further evaluation. PHYSICAL EXAMINATION He was a well developed male in no acute distress. There was no obvious rash or atrophy, though he had a mild effusion. Left knee range of motion was 0/3/130 degrees. He was diffusely tender with palpation, though mainly along the lateral joint line. There was no instability with Lachman's testing, anterior and posterior drawer testing and varus/valgus stress at 0 and 30 degrees. McMurray's test was negative. DIFFERENTIAL DIAGNOSIS Pigmented Villonodulous Synovitis Tuberculous arthritis Mycotic arthritis Monoarthritis secondary to Lyme Disease Gout/Pseudogout arthiritis TEST AND RESULTS Plain radiographs (AP, lateral and sunrise views of the knee): normal. MRI knee: Moderate effusion and chronic proliferate synovitis. Laboratory: HLA-B27, negative; RF positive; ANA with low titer of 1:40; ESR 93, CRP 4.4. CBC, chemistry, and LFT's were normal. Lyme serologies were positive. FINAL WORKING DIAGNOSIS Monoarthritis of the knee secondary to Lyme disease. TREATMENT AND OUTCOMES Doxycycline 100mg BID for 21 days. Continued home exercise program for quadriceps and hamstring strengthening. Complete resolution of pain, but some residual mild crepitus. Return to full activity by a four weeks.